There are many approaches to this. For example:
Generate a scene. It could be a painting, digital painting, 3D render, photo, or AI-generated.
You can find some video stock of rain falling. If it is on a green screen or black screen, that will depend on the blending mode, and the editing software used. The green screen footage will probably need to be processed differently from a normal green screen. Probably turned to grayscale.
But the water will not interact with the floor.
Modelate a floor on 3D software and project the texture on it. Separate other vertical elements like the walls as different objects. Define camera position.
Make a particle system using some physics so the droplets interact. Define if they interact with refraction or only with some shades.
Define light. Render.
Define the loop sections on the video.
Another approach could be filming a real-life scenario. You could need to embellish it with production lighting. You can then generate a loop.
There are some programs that make "moving effects". One is PhotoVibrance.io I have not used it and I don't know if it has a "rain" effect.
You then need to make a loop on other software.
But let me add my initial comment:
The most realistic answer is. Hire a designer. If you are doing a project, an app on mental health, you need mental health professionals, programmers for the app, and designers for the UI and animated scenery. You do not want the mental health professionals doing the programming, or the programmers doing the graphic design, while the designers are structuring the health problems. Even if it is a nonprofit app, you can find someone willing to help.